1. Field of the Invention
The present invention relates to an IPS (In-Plane Switching) mode liquid crystal display which rotates liquid crystal in a plane substantially parallel to a TFT substrate by applying a voltage across a pixel electrode and a common electrode formed in the TFT substrate, and more particularly to a liquid crystal display achieving enhanced contrast.
2. Description of the Related Art
In an active matrix type liquid crystal display, for example, thin film transistors (TFTs) are employed as pixel switching elements. Also, the active matrix type liquid crystal display attains a satisfactory image quality. For this reason, the active matrix type liquid crystal display is employed extensively as a monitor of a space-saving type desktop computer or the like. Generally, a liquid crystal display includes two operating modes: the twisted nematic (TN) mode, by which directors of aligned liquid crystal molecules are rotated in a direction perpendicular to the glass substrate, and the IPS mode, by which the directors are rotated in a direction parallel to the glass substrate.
In the IPS mode liquid crystal display, pixel electrodes and common electrodes, both having comb-teeth-wise portions extending parallel to each other, are formed on a transparent substrate for a TFT substrate. The TFT substrate is provided with TFTs. When a voltage is applied across the pixel electrode and common electrode, an electric field parallel to the substrate surface is developed. As a result, the orientation of the directors of the liquid crystal changes. A quantity of transmitting light is controlled by changing the orientation of the directors in this manner.
On the other hand, in case of the TN mode liquid crystal display, the directors of the liquid crystal may fall outside of the substrate surface and stand up. In such a case, a relation between a quantity of transmitting light and an applied voltage differs significantly when seen from the direction of the directors and from the normal direction of the substrate.
In the IPS mode liquid crystal display, as has been discussed, the directors rotate within plane of the substrate, and therefore, the above inconvenience does not occur. Hence, with the IPS mode liquid crystal display, a satisfactory image can be obtained in a very wide viewing angle.
The following description will describe behaviors of the liquid crystal molecules in the IPS mode with reference to FIG. 1 and FIGS. 2A and 2B. FIG. 1 is a view showing a relation of the orientation direction of the liquid crystal molecules between a pixel electrode 3 and a common electrode 2 versus an axis of polarized transmission. FIG. 2A is a view showing the orientation direction of the liquid crystal molecules in a dark state, and FIG. 2B is a view showing the orientation direction of the liquid crystal molecules in a light state.
In the IPS mode liquid crystal display, the liquid crystal is sandwiched by two polarizing plates having their respective axes of polarization intersecting at right angles, and as shown in FIG. 1 and FIGS. 2A and 2B, the liquid crystal is oriented homogeneously. The axis of polarization of one of the polarizing plates and the orientation direction of a liquid crystal molecule 10a are the same. Hence, black is displayed when no voltage is applied. Also, when a voltage across the pixel electrode 3 and common electrode 2 is applied, the liquid crystal molecule 10a is changed to twist toward the electric field, thereby displaying white. For this reason, the IPS mode liquid crystal display can lower luminance at displaying black in a stable manner.
In the conventional IPS mode liquid crystal display, however, the electric field is applied in the lateral direction, and therefore, transparent electrodes are not formed on the liquid crystal layer side of a color filter substrate (CF substrate) opposing the TFT substrate. For this reason, a color filter and a black matrix formed on the liquid crystal layer side of the CF substrate are not electrically isolated. Consequently, a charge distribution within the color filter is changed by an electric field applied to the liquid crystal, and an electric field is developed in the vertical direction, which disturbs the electric field in the lateral direction. Such disturbance of the electric field causes unwanted deterioration of the display quality, such as cross talks or image persistence.
Next, the following description will describe a change of the charge distribution within the color filter with reference to FIGS. 3 through 7. FIG. 3 is a plan view showing a constitution of a conventional IPS mode liquid crystal display. In the drawing, the layout of the electrodes and bus lines in the TFT substrate and the layout of the color filter in the CF substrate are superimposed. FIGS. 4 and 5 are cross sections taken along the lines A—A and B—B of FIG. 3, respectively. FIG. 6 is a view showing an equivalent circuit between a gate line and a pixel electrode. FIG. 7 is a view showing a correlation of the specific resistance of a black matrix versus luminance at displaying black in the conventional IPS mode liquid crystal display.
As shown in FIGS. 3 through 5, in the conventional IPS mode liquid crystal display, gate lines 6 and drain lines 5 intersecting substantially at right angles with each other are provided on a transparent substrate 1 for a TFT substrate 21, and a TFT 4 is provided to each intersection. Each pixel is provided with comb-teeth-wise pixel electrode 3 and common electrode 2. The pixel electrode 3 is connected to the TFT 4, and the common electrode 2 is connected to a common electrode line 2a that extends along the gate line 6. Further, a silicon nitride film 8 is formed to cover these electrodes. The longitudinal direction of the comb-teeth-wise portions of the pixel electrode 3 and common electrode 2 is substantially parallel to the drain lines 5. When a voltage is applied across the pixel electrode 3 and common electrode 2, an electric field is developed in a direction that intersects at substantially right angles with the longitudinal direction of the comb-teeth-wise portion and is substantially parallel to the surface of the transparent substrate 1.
A transparent substrate 14 for a CF substrate 22 is provided with a black matrix layer 13 for blocking incident light on the gate lines 6, drain lines 5, and a region between these lines and pixel display portions. Also, color layers 12 for color display, that is, three colors of RGB display, are formed on the transparent substrate 14. Further, an overcoat layer 11 is formed to cover the color layers 12.
An orientation film 9 is coated over the innermost surface of each of the TFT substrate 21 and CF substrate 22, and liquid crystal 10 is sandwiched between these substrates. The liquid crystal 10 is oriented homogeneously in a direction such that a predetermined angle is given with respect to the longitudinal direction of the pixel electrodes 3. A polarizing plate 15 is laminated to the outside of each substrate, and axes of polarization of the two polarizing plates 15 intersect at right angles with each other. The axis of polarization of one of the two polarizing plates 15 is set so as to be parallel to the orientation direction of the liquid crystal 10. All the common electrodes 2 are supplied with a constant common potential via the common electrode lines 2a. Each pixel electrode 3 is written with a potential from the drain line 5 through their respective TFTs 4. Consequently, an electric field is developed within the pixel, and the liquid crystal rotates with twisting, whereby the display is controlled.
In the conventional liquid crystal display, each color layer 12 is of a strip-wise shape connected to each other for a series of pixels provided in the extension direction of the drain lines 5 (the longitudinal direction in the drawing). For this reason, as shown in FIG. 5, each color layer 12 is formed to span the gate line 6 when seen from the normal direction of the substrate. Also, as shown in FIG. 4, the color layers 12 are separated above the drain lines 5, and parts of each color layer 12 overlap the drain line 5.
With the liquid crystal display arranged as above, the color layer 12 and black matrix layer 13 are in the electrically floating state, and capacitive-coupled to the gate lines 6, drain lines 5 and the like. Hence, when a signal voltage is applied on the gate line 6 or drain line 5, charges are injected into the color layer 12 and black matrix layer 13 at the portion overlapping the gate line 6 or drain line 5. Then, a vertical electric field is developed across the pixel electrode 3, and the color layer 12 and black matrix layer 13 as a potential of the color layer 12 and black matrix layer 13 changes. This disturbs the orientation of the liquid crystal molecules, and the luminance at displaying black is increased.
This phenomenon appears noticeably on the gate lines 6 having a larger voltage change. More specifically, in the conventional liquid crystal display, the color layers 12 are formed to cover the black matrix layer 13 above the gate lines 6. Thus, as shown in FIG. 6, the gate line 6 is connected to the pixel electrode 3 through a combined resistance of a parallel resistance, which includes a resistance R2 of the color layer 12 and a resistance R3 of the black matrix layer 13, and a resistance R1 of the liquid crystal 10. Therefore, in case that the color layer 12 or black matrix layer 13 has small resistance, a large volume of charges are injected into the color layer 12 or black matrix layer 13 from the gate lines 6, whereupon a large vertical electric field is developed across the color layer 12 and black matrix layer 13 and pixel electrode 3, which undesirably increases the luminance at displaying black remarkably. In general, a change in voltage on the gate line 6 is as large as ±20V approximately.
For example, FIG. 7 is a graph showing a relation of the specific resistance of the black matrix layer 13 versus luminance at displaying black. In general, the specific resistance of the color layer 12 is in the order of 1012 (Ω·cm), which is larger than that of the black matrix layer 13. For this reason, the parallel resistance of these layers largely depends on the specific resistance of the black matrix layer 13. Hence, as shown in FIG. 7, if the specific resistance of the black matrix layer 13 drops to the order of 1011 to 1010 (Ω·cm), the luminance at displaying black increases by a factor of approximately 7 from 0.5 (cd/cm2) to 3.5 (cd/cm2).
For example, contrast is used as an indicator representing the performance of the liquid crystal display, which is a ratio of the luminance at displaying black with respect to the luminance at displaying white. Because the luminance at displaying white changes little, the contrast largely depends on the luminance at displaying black. For instance, in the above-described case, the contrast is dropped to {fraction (1/7)}. Hence, in order to enhance the contrast, it is important to lower the luminance at displaying black. In other words, it is important to reduce unwanted vertical electric fields developed across the color layer 12 and black matrix layer 13 and the pixel electrode 3 by reducing a quantity of charges injected into the color layer 12 and black matrix layer 13 by taking a countermeasure, such as using a high specific resistance material.
However, the specific resistance of the color layer 12 and black matrix layer 13 has an upper limit. Also, there is a problem that a range of material selection is narrowed if the specific resistance is increased. Therefore, there has been an increasing need for a liquid crystal display capable of reducing injected charges while securing a large tolerance for the specific resistance of the color layer 12 and black matrix layer 13.